There is increasing interest in energy harvesting circuits, such as for use in miniature wireless devices to enable them to operate autonomously for extended periods of time. The harvested energy may be used to charge a battery, although batteryless circuits can also be operated based on harvested power.
It is known that an inductive DC-DC converter architecture can form the main building block of an energy harvesting circuit.
FIG. 1 shows three of the most common inductor based DC-DC converter topologies as applied to energy harvesting in wireless applications. FIG. 1(a) shows a buck converter, FIG. 1(b) shows a boost converter, and FIG. 1(c) shows a polarity inverting buck-boost converter.
Each circuit is supplied by a generated voltage VIN across which an input capacitor C1 is connected. A transistor Q1 controls the completion of a first circuit in which the generated voltage is placed in series with an inductor L. The transistor Q1 is used to ramp up current in the inductor L with a slope that will be dependent on the input voltage VIN.
A diode D1 defines an output circuit with the inductor L. The diode is used to transfer the energy to the load, until the inductor current reaches a zero level.
The output VOUT is smoothed by an output capacitor C2 and may for example be stored by an energy storage element.
The switching of the transistor Q1 in each case controls the DC-DC voltage conversion operation. The operation of these circuits is well known and will not be described in further detail. Each converter topology can be run continuously at higher input power levels, or operated in a pulsed discontinuous mode at lower power levels.
FIG. 1 shows the most basic circuit implementations using a single switch and a single diode. In practice, the diodes can be implemented as active components using transistors and a comparator to drive the transistor at the appropriate times. Active diode implementations can be used to limit losses. Furthermore, additional circuit elements may be used, for example to make integrated circuit implementation easier. By way of example, FIG. 2 shows a buck-boost converter suitable for implementation using an integrated circuit. It can be seen that the circuit essentially combines the input switching circuit of the buck converter of FIG. 1(a) with the output switching circuit of the boost converter of FIG. 1(b). It has two switching transistors Q1, Q2 and two diodes D1, D2.
Improvements in energy harvesting technology, for example using the types of converter circuit shown in FIG. 1, have enabled a series of applications, in which the battery used to power sensors or transducers does not need to be replaced, but can be recharged using environmental energy. However, in many applications power extracted by means of energy harvesters is very small (from a fraction of a μW to a few tens of μW).
If the equivalent resistance of the harvester can be considered almost constant for the entire range of delivered powers (as in thermoelectric generators (TEGs) and sometimes in RF and vibrational harvesters), the power management interface can be designed in order to behave as a fixed resistance.
An open-loop inductive DC-DC converter can be modeled as a constant resistance under the following conditions:Buck: VIN>>VOUT=>RIN=2TL/TON2 Boost: VIN<<VOUT=>RIN=2TL/TON2 Buck-Boost: RIN=2TL/TON2 where VIN is the input voltage, VOUT is the output voltage, RIN is the input resistance, T the conversion period, L the inductance, and TON the interval of time in which the inductor is magnetized. So, if the assumptions indicated above are fulfilled, the converter input resistance is independent of input and output voltage.
This approach can be used to avoid the implementation of a Maximum Power Point Tracking (MPPT) algorithm for maximizing the output power, because, after an initial calibration, the power management unit behavior would not need to be changed anymore. This is for example explained in the article “Resistor Emulation Approach to Low-Power Energy Harvesting”, IEEE PESC pp. 1-7, June 2006, of Paing T. S., et. al. This approach can however also be used together with an MPPT algorithm, allowing to dramatically lower the required tracking speed of the MPPT, because fast input and output voltage changes would not affect the impedance matching with the source.
The article “Resistive matching with feed-forward controlled non-synchronous boost rectifier for electromagnetic energy harvesting”, IEEE APEC, pp. 3081-3086, March 2013 by P. P. Proynov et. al. discloses a boost converter control loop for automatically adjusting the conversion frequency in a Pulse-Frequency-Modulation (PFM) scheme in order to keep the input resistance constant.
The article “Energy Harvesting for Autonomous Systems”, Artech House Publishers, pp. 183-187, 2010, ISBN: 9781596937185 by P. D. Mitcheson et. al. discloses a boost converter control loop for controlling the input resistance in a Pulse-Width-Modulation (PWM) scheme.
The PFM and PWM schemes proposed in the prior art are valuable when large input powers are available, but become unfavorable when input powers are remarkably low.